Conventionally, in disc devices that play back data from or record data to disc storage media, such as optical discs and magneto-optical discs, into which various information such as video information, audio information, and computer data is written in the form of an radio-frequency signal, a servo control is performed for keeping an optical pickup in a desired position. Some examples of such servo control are a tracking control for moving the optical pickup along a track of a disc storage medium, and a focusing control for setting the focus of a laser beam emitted from the optical pickup toward the disc storage medium.
To perform the tracking control and focusing control described above, the disc device is provided with an arithmetic processing circuit unit that generates a tracking error signal and a focusing error signal based on the laser beam received by the optical pickup. Such a disc device is offered that is provided with an arithmetic processing circuit unit that can generate different tracking error signals according to a 3-beam method and a phase difference detection method (see Patent Document 1).
In a conventional arithmetic processing circuit unit of a disc device, a circuit for a subtraction operation, for example, is configured as shown in FIG. 10. This configuration includes filter circuits 101 and 102 provided one for each of two input signals for permitting a signal at a predetermined frequency to pass therethrough, a subtraction circuit 103 performing a subtraction operation on the two signals that have passed through the filter circuits 101 and 102, and an offset adjustment circuit 104 removing an offset from the signal obtained as a result of the subtraction performed by the subtraction circuit 103.
In this case, the filter circuits 101 and 102 are each built with an amplifier A, and a capacitor Cx and a resistor Rx, each being connected at both ends thereof to the input and output of the amplifier A. The subtraction circuit 103 is built with an operational amplifier A1 having a non-inverting input terminal and an inverting input terminal connected to the outputs of the filter circuits 101 and 102. The offset adjustment circuit 104 is built with an operational amplifier A2 having a non-inverting input terminal connected to the output of the subtraction circuit 103 and an inverting input terminal to which an offset voltage is inputted.
Patent Document 1: JP-A-2002-008232